1. Technical Field
The present invention relates to a switching power supply device which converts an input voltage to a pulsed voltage by a switching operation and rectifies this pulsed voltage to output a direct-current (DC) voltage.
2. Related Art
For example, an electric car or a hybrid car is equipped with a high-voltage battery and a power supply device. The high-voltage battery is used for operating a driving motor. The power supply device lowers a voltage from the battery to supply to various in-car electric components. Generally, a switching power supply device (DC-DC convertor) is used as the power supply device. The switching power supply device includes a switching circuit, a rectifying circuit and a smoothing circuit. The switching circuit converts a DC voltage to a pulsed voltage by a switching operation according to a pulse width modulation (PWM) signal. The rectifying circuit rectifies the pulsed voltage. The smoothing circuit smooths the voltage rectified by the rectifying circuit. This switching power supply device is disclosed in Japanese Unexamined Patent Publication Nos. 2008-236999, 2003-274648, and 2002-305873 which will be described later.
The switching power supply device requires a lot of sensors detecting an input voltage, an input current, an output voltage and an output current, etc., in order to constantly monitor states of circuits. In order to detect an input voltage, a resistor having a large allowable power value is required for dividing the input voltage, and hence a large space for circuit implementation is needed. A current sensor or a shunt resistor, etc., may be used as means for detecting an input current or an output current. However, there are drawbacks that the current sensor has a complex configuration, and the shunt resistor has a large power loss. In contrast, if a current transformer is used as the current detecting section, there is an advantage that a circuit configuration can be simplified. The current transformer is used in the switching power supply device as disclosed in Japanese Unexamined Patent Publication Nos. 2008-236999, 2003-274648, and 2002-305873.
In the switching power supply device disclosed in Japanese Unexamined Patent Publication No. 2008-236999, a current detecting part includes a current transformer and a sample and hold part holding a peak value of a signal obtained by the current transformer to output it as a detection signal. A control part provides a driving signal having a predetermined pulse width to a switching device, based on the detection signal from the current detecting part and an output voltage. The control part estimates an average value with respect to primary current ripple based on the output voltage and the driving signal applied to the switching device, and calculates a value of an output current through a load based on the average value.
In the switching power supply device disclosed in Japanese Unexamined Patent Publication No. 2003-274648, a current detecting part includes a current transformer detecting a current of a switching section as a voltage, and a voltage converting circuit converting a duty rate of a driving pulse for the switching section to a voltage. The current detecting part adds the converted voltage by the voltage converting circuit to the detected voltage by the current transformer, and outputs it to a PWM control part as a corrected detection voltage.
The switching power supply device disclosed in Japanese Unexamined Patent Publication No. 2002-305873 includes an overcurrent protective section which detects a peak current of current through a switching section by a current transformer to limit an output current. In the overcurrent protective section, the detected peak current value is corrected by using an input voltage, an output voltage and a voltage proportional to a duty rate of the switching section.
Regarding the switching power supply device, one microcomputer may include a PWM signal generating part for generating a PWM signal and a control part for controlling an output voltage. In this case, since a PWM process and a control process have to be concurrently performed in the same microcomputer, there is a problem that the PWM process slows down due to the control process.
If the PWM signal generating part for generating the PWM signal and the control part for controlling the output voltage are included in different microcomputers, respectively, the PWM process and the control process are individually performed, and hence the PWM process can be accelerated. For example, if a process cycle of the microcomputer for the control is set to 1 millisecond and a process cycle of the microcomputer for the PWM is set to 10 microseconds, the process cycle of the microcomputer for the PWM can be shortened, and hence an operating frequency of the switching circuit can be increased. Accordingly, since the switching circuit rapidly operates when a failure is detected, a response can be improved. In addition, a transformer connected between the switching circuit and the rectifying circuit can be downsized.